The present invention relates to a very-small-signal amplifier apparatus and a magnetic disk storage apparatus using the same. More specifically, the invention relates to a magnetic disk storage apparatus such as a hard disk drive and, particularly, to a technology that can be effectively utilized for amplifying very small signals by using a magnetoresistance effect element (MR) head as a read head and, particularly, to a technology that can be effectively utilized for amplifying very small signals by using a composite head including a magnetoresistance effect element as the read head and an inductive head as a write head.
A read amplifier for MR head constituted by CMOS (complementary MOS) has been disclosed in IEEE Journal of Solid State Circuits, Vol. 29, No. 12, December, 1994, pp. 1589-1595. According to the read amplifier disclosed in the above-mentioned literature, a bias current is supplied to an MR head through an amplifier MOSFET to generate take out a read current that varies depending upon a change in the magnetoresistance.